trace_processor: add import on stddef.h in stats.h
trace_processor: add import on stddef.h in stats.h

Change-Id: I778d4ca8ff01cf211bc00c57d468ac61206c10c4
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto